In microdiffraction analysis, an electron beam is directed towards a crystalline sample and the interaction of the electrons in the electron beam with the sample causes different types of particle to be produced. Two types of particle are of particular interest, these being backscattered electrons and X-rays. X-rays produced within the sample and diffracted by the lattice planes of the crystal are used in Kossel X-ray analysis, whereas electrons originating from the source electron beam elastically backscattered from the sample and having an energy close to the primary beam energy of the beam form the basis for Electron Back Scatter Diffraction analysis. These two analysis techniques can be used to measure properties of the sample, for example crystal orientation and strain.
In Kossel X-ray diffraction analysis, a focussed electron beam is directed towards a sample, typically in a scanning electron microscope (SEM). The interaction of the electron beam with the sample produces X-rays which are diffracted by the lattice planes of the sample. The “Kossel diffraction pattern” produced by the X-rays can be used to analyse the crystal orientation and other properties of the sample. The Kossel “signal” is a weak (low intensity), divergent beam of element-characteristic, diffracted X-rays emitted from the sample along with strong (high intensity) “background” beams of other particles, which are detected by conventional apparatus such as a photographic film or a solid-state electronic camera. Due to the “background” beams, the signal to background ratio is very small in SEM Kossel analysis, approximately 2%. The “background” beams have both undiffracted and diffracted components comprising different element-characteristic X-rays, X-rays forming a continuous energy spectrum, back-scattered electrons (with an approximately continuous energy spectrum), and UV to IR photons.
In order to improve the signal to background ratio in Kossel analysis a filter is typically placed between the sample and camera which absorbs the background beams while allowing the Kossel X-rays to pass through. The optimum filter composition and structure may be sample-dependent as the characteristic X-ray energy differs between samples. Example filters include a free-standing Al, Be, Fe or Ni film (or composite structures combining more than one of these elements); a film integrated as a window into the vacuum interface between the SEM vacuum chamber and camera; or a film deposited directly onto the camera.
The actual choice of filter used is a compromise between practicality, function and cost, and properties to consider include material, thickness, area and positioning in relation to the source and camera. The need for an absorption filter remains an inconvenient complication to the practical operation of the analysis.
A further complication with Kossel analysis is that the detected diffraction patterns are often complex, for example those from compound materials may be a superposition of patterns, wherein each different element of the compound contributes a different pattern dependent on the energy of its characteristic X-rays. Such superposed patterns are difficult to analyse. The conventional “pseudo” Kossel technique attempts to overcome this problem by inserting a single element foil between the electron source and the sample in order to provide a local source of monochromatic X-rays, rather than the X-ray source being within the sample itself. Alternatively, a coating may be used on the sample instead of a foil.
However, “pseudo” Kossel analysis has significant disadvantages, for example reduced spatial resolution and inconvenient complication to the practical operation of the apparatus. Further, where a coating is used instead of a foil, this can cause undesirable changes to the sample itself.
Electron Back Scatter Diffraction (EBSD) is a complementary technique to Kossel analysis. In EBSD, similarly to Kossel, a focussed electron beam is directed towards a sample, typically in a SEM. Here, the signal of interest is low-loss, back-scattered, diffracted electrons with energy close to the primary energy of the source electron beam, rather than the diffracted X-rays in Kossel analysis. These low-loss electrons are typically analysed in the form of a “Kikuchi diffraction pattern”. However, as well as diffracted electrons from the sample, there are also background beams such as X-rays and low-energy electrons. Unlike with Kossel analysis, it is not possible to construct an absorption filter which will allow the transmission of the signal electrons whilst blocking the background X-rays and other background particles; and the conventional approach is to use no filter and accept that the signal to background ratio for EBSD is very low.
Hooghan et al, Microsc. Microanal. 10(Suppl 2), 2004, discloses a possible approach to increasing the signal to background ratio for EBSD by using an electrostatic element as a filter. This allows electrons to be filtered according to their energy, for example rejecting some lower-energy, background electrons. Even so, X-rays, being uncharged, are not filtered and so these still contribute to the background.
There are further disadvantages to such a filter. For example, the filter reduces the solid angle of detection, so reducing the angular range of the Electron Back Scatter Pattern (EBSP). There is also the requirement to operate with high voltages which complicates interfacing with an SEM; the filter is bulky, requiring more space inside the SEM chamber and the filter operates effectively only in a narrow, high-pass mode (imaging aberrations are large unless operated very close to the SEM's primary beam energy).
It is therefore desired to improve the filters used in both Kossel X-ray and EBSD analysis to increase the signal to background ratio and to facilitate simpler operation of the apparatus. Additionally, EBSD and Kossel analysis techniques are complementary and are beneficially both carried out on the same sample to measure crystal orientation, strain and other properties. However, this is currently a time-consuming and difficult process to carry out due primarily to the requirement of the absorption filter in Kossel analysis. If the EBSD analysis is desired to be carried out using an electrostatic filter then this further complicates the procedure. For example, if a sample is to be analysed in a SEM by both Kossel and EBSD techniques then the SEM may first be run with an ultra-thin Be filter in place in order to produce a Kossel diffraction pattern. For EBSD to subsequently be carried out on the same sample, the filter must first be removed, meaning downtime of the SEM. This removal (or insertion if EBSD is performed first) of the filter is particularly disadvantageous when a specific region of interest on the sample has been located for analysis and must be re-located between procedures. Additionally, “pseudo” Kossel analysis where the sample is coated is incompatible with EBSD analysis.
In U.S. Pat. No. 6,462,340, provision is made for a dual purpose device with moveable filter such that a single apparatus can be used for both the detection of EBSD and Kossel patterns. However, this still requires the movement of an absorption filter. Further filter properties such as its material and thickness may need to be changed depending on the particular application or sample.